A dual connectivity framework is currently being considered for LTE Rel-12. Dual Connectivity (DC) refers to operations where a given UE consumes radio resources provided by at least two different network points (Master eNB, MeNB; and Secondary eNB, SeNB) connected with non-ideal backhaul while in RRC_CONNECTED. A UE in DC maintains simultaneous connections to anchor and booster nodes, where the MeNB is interchangeably called anchor node and the SeNB is interchangeably called booster node. As the name implies, the MeNB controls the connection and handover of SeNB. No SeNB standalone handover is defined for Rel-12. Signaling in MeNB is needed even in SeNB change. Both the anchor node and booster node can terminate the control plane connection towards the UE and can thus be the controlling nodes of the UE.
The UE reads system information from the anchor. In addition to the anchor, the UE may be connected to one or several booster nodes for added user plane support. The MeNB and SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
More specifically DC is a mode of operation of a UE in RRC_CONNECTED state, where the UE is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB.
A Master Cell Group (MCG) is a group of serving cells associated with the MeNB, including the PCell and optionally one or more SCells. A Secondary Cell Group (SCG) is a group of serving cells associated with the SeNB including the pSCell (Primary Scell) and optionally one or more SCells. A master eNB is the eNB which terminates at least S1-MME. A secondary eNB is the eNB that is providing additional radio resources for the UE but is not the Master eNB.
FIG. 1 illustrates a dual connectivity deployment configuration system having a plurality of MeNBs, SeNBs, and UEs. In this example, only one SeNB is connected to a UE, however, more than one SeNB can serve the UE in general. FIG. 1 also illustrates that dual connectivity is a UE specific feature and a network node can support a dual connected UE and a legacy UE at the same time.
As explained above, the anchor and booster roles are defined from a UE point of view. This means that a network node that acts as an anchor to one UE may act as a booster to another UE. Similarly, although a UE reads the system information from the anchor node, a network node acting as a booster to one UE may or may not distribute system information to another UE.
The terms anchor node and MeNB are used herein interchangeably. Similarly, the terms SeNB and booster node are used herein interchangeably.
The MeNB operates to provide system information, terminate the control plane of the layered protocol structure between a UE and a network node, and may also terminate the user plane. The SeNB operates to terminate the control plane or may operate to terminate only the user plane.
In one application, dual connectivity allows a UE to be connected to two network nodes to receive data from both network nodes and, thereby, increase the UE's data rate. This user plane aggregation may achieve similar benefits as carrier aggregation using network nodes that are not connected by a low-latency backhaul/network connection. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the UE to each of the network nodes will need to be performed separately. That is, it's expected that the UE will have two UL transmitters to transmit UL control and data to the connected network nodes.
Synchronized or Unsynchronized Dual Connectivity Operation
Since dual connectivity (DC) operation involves two non-co-located transmitters (i.e. MeNB and SeNB), one of the main issues related to UE receiver performance is the maximum receive timing difference (Δt) of the signals from MeNB and SeNB received at the UE receiver. This gives rise to two cases of DC operation with respect to the UE: Case (1)—synchronized DC operation, and Case (2)—unsynchronized DC operation.                Synchronized operation used herein means that the UE can perform DC operation provided the received time difference (Δt) between the signals received at the UE from the component carriers (CCs) belonging to the MCG and SCG are within a certain threshold, e.g. ±30 μs. As a particular non-limiting example, the synchronized operation herein means that the received time difference (Δt) between the signals received at the UE from the subframe boundaries of the CCs belonging to the MCG and SCG are within a certain threshold, e.g. ±30 μs.        Unsynchronized operation used herein means that the UE can perform DC operation regardless of the received time difference (Δt) between the signals received at the UE from the CCs belonging to the MCG and SCG i.e. for any value of Δt. As a particular non-limiting example, the unsynchronized operation herein means that the received time difference (Δt) between the signals received at the UE from the subframe boundaries of the CCs belonging to the MCG and SCG can be any value, e.g. more than ±30 μs, any value up to ±0.5 ms etc.        
Maximum receive timing difference (Δt) at the UE can have the following components:                (1) Relative propagation delay difference between MeNB and SeNB,        (2) Tx timing difference due to synchronization levels between antenna connectors of MeNB and SeNB, and        (3) Delay due to multipath propagation of radio signals.Relative Propagation Delay Difference        
A maximum of 30.26 μs relative propagation delay corresponds to a worst case non-co-located CA coverage case, where the signal propagation distance is just over 9 km. In dense urban scenarios, maximum misalignment due to propagation delay that can be seen can be about 10 μs. The propagation delay is linearly related to relative physical distance between the network nodes. Consequently, there is a large amount of timing misalignment margin which may not be required due to distance between network nodes, and which means that there is a possibility to relax the requirement beyond certain transmit timing misalignment (i.e. synchronization accuracy between MeNB and SeNB), e.g. 3 μs. The 3 μs time is chosen here due to the co-channel synchronization accuracy requirement for TDD systems being 3 μs (which means that the tightest requirement that can be achieved is 3 μs).
Transmit Timing Difference Between MeNB and SeNB
The synchronized case essentially means that MeNB and SeNB transmit timing need to be synchronized up to a certain level of time accuracy, while the unsynchronized case provides a random value for synchronization accuracy (e.g., anything up to 1 ms), which is higher than the accuracy required in the synchronized case. It is noted that the receive timing difference is the received timing misalignment between two received signals at the UE or, in other words, is not the transmit timing mismatch levels between the MeNB and SeNB.
FIG. 2 illustrates the maximum receive timing difference at the UE. As the baseline option, it is assumed that dual Tx/Rx is used with a non-ideal backhaul, and that the MeNB and SeNB are not synchronized to each other. Dual Tx/Rx means that there can be separate PAs for separate links, such that no strict synchronization requirement is needed, and which is the case (2) explained above. Requirements defined for the un-synchronized case will also work for the synchronized case. However, considering the implementation and requirements issues for synchronized and unsynchronized dual connectivity operation, the following embodiments are provided for dual connectivity operation. Case (1) described above suggests defining certain synchronization accuracy between MeNB and SeNB.
Delay Due to Multipath Radio Environment
The received time difference of radio signals from MeNB and SeNB may also incorporate additional delay introduced by the multipaths due to the characteristics of the radio environment. For example, in a typical urban environment the delay spread of multiple paths received at the UE may be in the order of 1-3 μs. In contrast, for wide areas, such as sub-urban or rural deployment environments, the multipath effect on channel delay spread of signals observed at the UE can be relatively smaller, e.g. less than 1 μs.
Dual Connectivity is a UE Specific Operation
In general, network-wide synchronization is not needed for dual connectivity since dual connectivity is a UE specific operation. A UE can be connected to two eNBs in dual connectivity operation, thus the synchronization requirement is needed between only two eNBs when they serve the UE for dual connectivity operation, i.e. the involved MeNB and SeNB. It is noted that the same MeNB and SeNB may also be serving UEs not in dual connectivity. Thus, no synchronization requirements, even between MeNB and SeNB, are specified. However to ensure that the UE operating in dual connectivity is able to receive signals from MeNB and SeNB within the maximum allowed received time difference, the following conditions related to the involved eNBs are defined for the UE to meet:                1. The received time difference at the UE from the MeNB and the SeNB is within the allowed limit; and        2. The maximum transmit time difference between the MeNB and the SeNB is within certain time limit.RRM Measurement        
Several radio related measurements (RRMs) are used by the UE or the radio network node to establish and keep the connection, as well as to ensure the quality of a radio link.
The UE has to first detect a cell and therefore cell identification, e.g. acquisition of a physical cell identity (PCI), which is a signal measurement. The UE may also have to acquire the cell global ID (CGI) of a UE.
The UE reads the system information (SI) of the target cell (e.g., intra-, inter-frequency or inter-RAT cell) upon receiving an explicit request from the serving network node via radio resource control (RRC) signaling, e.g. from RNC in HSPA or eNode B in case of LTE. The acquired SI is then reported to the serving cell. The signaling messages are defined in the relevant HSPA and LTE specifications.
In order to acquire the SI which contains the cell global identifier (CGI) of the target cell, the UE has to read at least part of the SI including a master information block (MIB) and the relevant system information block (SIB) as described later. The terms SI reading/decoding/acquisition, CGI/ECGI reading/decoding/acquisition, and CSG SI reading/decoding/acquisition are interchangeably used herein and may have the same or similar meaning.
The reference signal received power (RSRP) and reference signal received quality (RSRQ) are the two existing measurements used for at least RRM such as for mobility, which include mobility in RRC connected state as well as in RRC idle state. The RSRP and RSRQ are also used for other purposes such as for enhanced cell ID positioning, minimization of drive test, etc. Other examples of UE measurements are UE Rx-Tx time difference measurement, reference signal time difference (RSTD), etc.
In RRC connected state the UE can perform intra-frequency measurements without measurement gaps. However as a general rule the UE performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE:                Measurement gap pattern #0 with repetition period 40 ms; and        Measurement gap pattern #1 with repetition period 80 ms.        
The measurements performed by the UE are then reported to the network for use in various tasks.
The radio network node (e.g. base station) may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between a UE and itself, uplink (UL) signal-to-interference-plus-noise (SINR), UL signal-to-noise ratio (SNR), UL signal strength, Received Interference Power (RIP), timing advance (TA), eNode Rx-Tx time difference measurement, etc. The eNB may also perform positioning measurements which are described further below.
The UE also performs measurements on the serving cell (e.g., primary cell) in order to monitor the serving cell performance. This is referred to as radio link monitoring (RLM) or RLM related measurements in LTE.
For RLM the UE monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving or PCell.
In order to detect out of sync and in sync the UE compares the estimated quality with the thresholds Qout and Qin respectively. The threshold Qout and Qin are defined as the level at which the downlink radio link cannot be reliably received and corresponds to 10% and 2% block error rate of a hypothetical PDCCH transmissions respectively.
Potential Problems with Existing Approaches
Currently, measurement gap length is defined independently for each CG in dual connectivity. This means that, when SFN synchronization is not assumed in dual connectivity, the UE will not know on which subframes to enforce the MGL for SCG with respect to MGL that is configured in MCG.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.